Process for producing one- and two-ring aromatics from polynuclear aromatic feedstocks

ABSTRACT

In a process for upgrading a topped-tar fraction of coal tar, topped-tar feedstock containing largely polynuclear aromatic hydrocarbons is hydrocracked in a hydrogen atmosphere in a first reaction zone at a temperature and pressure sufficient to hydrocrack the large molecules to three- and two-ring aromaticnaphthenes. The hydrocracked feed which also contains olefins, is catalytically hydrotreated in a second reaction zone to saturate the olefins, and the feed is then catalytically treated in a third reaction zone at a temperature, pressure and hydrogen to hydrocarbon feed mol ratio sufficient to hydrocrack the threeand two-ring compounds to two- and one-ring aromatics, while, at the same time, naphthenes are dehydrogenated to aromatics, the alkyl aromatics are hydrodealkylated to nonsubstituted aromatic compounds, and substantially all oxygen, nitrogen and sulfur compounds unreacted in the first and second treatment zones are hydrorefined.

United States Patent 2,958,643 ll/l960 Friedman Inventor Mehmet Orhan Tarhan Bethlehem, Pa.

App]. No. 879,641

Filed Nov. 25, 1969 Patented Nov. 30, 1971 Assignee Bethlehem Steel Corporation PROCESS FOR PRODUCING ONE- AND TWO- RING AROMATICS FROM POLYNUCLEAR AROMATIC FEEDSTOCKS 10 Claims, 2 Drawing Figs.

1.8. CI 208/60, 208/79. 208/l08, 208/136, 208/210, 208/211, 208/216. 260/672 R RECYCZE spa/r62 3,197,5[8 7/l965 Chapmanetal 3,317,622 5/1967 Hoertzetal.

ABSTRACT: In a process for upgrading a topped-tar fraction of coal tar, topped-tar feedstock containing largely polynuclear aromatic hydrocarbons is hydrocracked in a hydrogen atmosphere in a first reaction zone at a temperature and pressure sufficient to hydrocrack the large molecules to threeand two-ring aromatic-naphthenes. The hydrocracked feed which also contains olefins, is catalytically hydrotreated in a second reaction zone to saturate the olefins, and the feed is then catalytically treated in a third reaction zone at a temperature, pressure and hydrogen to hydrocarbon feed mol ratio sufficient to hydrocrack the threeand two-ring compounds to twoand one-ring aromatics, while, at the same time, naphthenes are dehydrogenated to aromatics, the alkyl aromatics are hydrodealkylated to nonsubstituted aromatic compounds, and substantially all oxygen, nitrogen and sulfur compounds unreacted in the first and second treatment zones are hydrorefined.

EE/YZENE RECYCLE PATENTED nuvao Ian SHEET 1 or 2 w uxuwk u u convert high molecular weight aromatic compounds to two- 10 and three-ring compounds. This partially hydrocracked product will nonnally include aromatics, alkyl aromatics, naphthenes, olefins, and certain sulfur, oxygen or nitrogen compounds, and this product can be hydrotreated in a second reactor step where the olefins are saturated. The efi'luent from the second reactor is fractionally distilled to separate twoand three-ring compounds from compounds having in excess of three rings. The latter compounds are recycled to the first reactor step, while the remainder, a portion boiling above about 325 C. is transferred to a stripping operation, where it 2 is freed of some sulfur, nitrogen and C, to C hydrocarbons in the form of gaseous products. Liquid effluent from the stripping operation, containing the twoand three-ring aromatics and naphthenes, enters a third reaction step, where it is treated catalytically in the presence of hydrogen under controlled conditions of temperature, pressure, liquid hourly space velocity, and hydrogen flow rate. Efiluent from the third reactor step contains principally benzene, toluene, xylene, C to C aromatics and naphthalene.

The chemical oil fraction and the C to C, fraction of coal tar can be supplied directly to the second reactor step, along with the hydrocracked topped far from the first reactor step, as will be described more fully below.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic representation of a preferred method of hydroupgrading coal tar.

FIG. 2 is a modification of FIG. 1.

DETAILED DESCRIPTION As mentioned above, this invention can be performed on the topped-tar fraction of coal tar alone, or on each of the principal coal tar fractions collectively, i.e. topped tar, chemical oil and light oil, in either a batch or continuous operation.

A preferred mode of performing the invention will be given below, wherein the crude coal tar, obtained from carbonization of bituminous coal, is utilized in a continuous process.

Referring to FIG. 1 of the drawing, crude coal tar, obtained from coke ovens, is introduced into a fractionator 12 from line 11. In the fractionator, the tar is separated into three fractions comprising a portion of the light oil fractionthe benzenetoluene (BT) fractionand water, a chemical oil and C to C fraction, including, naphthalene, methyl naphthalenes and phenol, and the topped-tar fraction as residue containing the high molecular weight compounds. These high molecular weight compounds include three-ring compounds, such as anthracene, phenanthrene, fluorene, acenaphthene and carbazole; four-ring compounds such as chrysene, pyrene, fluoranthene; and higher ring compounds containing up to 30-40 rings per molecule. The topped tar is withdrawn from the bottom of fractionator 12 by way of line 13, and by means of pump 14 is pumped to a pressure of 3,000 p.s.i.g. Fresh hydrogen from line 17 is compressed in compressor 15, also to a pressure of 3,000 p.s.i.g., and is heated by flowing through a convection coil in heater l6. Leaving the heater by line 18, the hydrogen is mixed with the pressurized tar stream from compressor 14 at a point on line 19, whence the mixture enters heater 16, and is heated to 450 C. The hydrogen must be heated prior to mixing with the tar in order to prevent freezing of the tar in the mixing line. The heated tar and hydrogen mixture is transferred by way of line 21 to the first step reactor 22.

In reactor 22, topped tar is hydrocracked over a sulfided 1 iron-molybdate on alumina catalyst at a liquid hourly space velocity (LI-ISV) of 0.6 vol./(hr.) (catalyst vol.), and a hydrogen to liquid feed volume ratio of 965. A hydrocracked product, which will, for convenience, be referred to as hydrocrackate," containing about 35 percent by weight of compounds boiling below 325 C. is produced. The overhead effluent from reactor 22 flows through line 23 to hydrotreat reactor 24.

The chemical oil, C to C fraction, leaves fractionator 12 by way of line 41 to pump 40 and is then combined with the hydrocrackate effluent in line 23. The chemical oil, C to C fraction, is used to cool the reactor 22 effluent so that the resultant mixture has the proper temperature for step 2 reaction in reactor 24. In case larger amounts of the chemical oil, C to C fraction, are mixed with the hydrocrackate, thus cooling the mixture below the desired temperature, the inlet temperature in reactor 24 can be maintained by adding hot process hydrogen at line 43. Process hydrogen is a mixture of fresh hydrogen and hydrogen recovered from the spent hydrogen streams, and contains about 5 percent to 20 percent 0 of C to C hydrocarbon gases.

In the hydrotreat reactor, the mixture of hydrocrackate, chemical oil and C to C aromatics is reacted with hydrogen over a cobalt-molybdate on alumina catalyst at a temperature of about 400 C., a pressure of about 3,000 p.s.i.g. and a high space velocity. The hydrotreat reaction is performed to saturate olefins, such as styrene, indene and coumarone, and to thus prevent formation of polymers of these compounds, which tend to clog equipment when exposed to heated surfaces, and to hydrogenate oxygen-containing compounds such as phenol, cresols and xylenols by forming water and aromatics therefrom. Oxygenated compounds and part of the olefins are contained in the chemical oil, C to C fraction, and substantial amounts of olefins are formed in the first step reactor.

The effluent from the hydrotreat reactor 24 flows through condenser 26 by way of line 25, thence to high-pressure flash drum 27, where the liquid condensate separates from noncondensible gases at about 40 C. and full system pressure, a little below 3,000 p.s.i.g. Water condensate is drained intermittently from water trap 28. Spent hydrogen is bled, via line 29 and valve 29' to a hydrogen purification system, not shown. The condensed hydrotreater effluent is sent, via line 30 and valve 30, to low-pressure flash drum 31, where most of the 5 gases which were held in solution at 3,000 p.s.i.g. are released at a pressure between 150 and 200 p.s.i.g. These gases are conveyed by way of line 33 and valve 33 to the low-pressure side of line 29. The hydrotreater effluent flows from the lowpressure flash drum via line 32 and valve 32 through line 34 and heat exchanger 35 to fractionator 36, where the feed is separated into an overhead distillate fraction boiling below 325 C. and a residue boiling above 325 C. The residue is recycled by way of line 37 and compressor 38 to line 19, and then to reactor 22. The distillate flows to stripper 39, by way of line 45, where residual gases including hydrogen sulfide, ammonia and C to C aliphatic hydrocarbons are removed overhead by way of line 49. Bottoms from stripper 39 are withdrawn via line 57, compressed by compressor 46 to a pressure in excess of 800 p.s.i.g., blended with compressed process hydrogen from line 52 and passed through heater 61 where they are heated to 610 C., and are then transported via line 62 to reactor 63.

The water (about 2 percent) contained in the coal tar feed is separated from the distillate in the fractionator and removed at line 48.

The portion of the light oil boiling below C. leaves the fractionator as overhead at line 42. This portion contains crude benzene and toluene (BT Fraction), and requires no hydrotreatment. The BT fraction is pumped in pump 44 to a pressure of about 800 p.s.i.g., after which it is joined with the hydrotreater distillate in line 47. Thus, the three fractions of the coal tar from fractionator 12 are joined in line 47, and after mixing with process hydrogen and preheating in heater 61 to 610 C., the combined fractions are introduced by way of line 62 into third step reactor 63. These combined fractions PROCESS FOR PRODUCING ONE- AND TWO-RING AROMATICS FROM POLYNUCLEAR AROMATIC FEEDSTOCKS BACKGROUND OF THE INVENTION This invention relates to an improvement in converting polynuclear (two-ring and larger) aromatic hydrocarbons to low molecular weight aromatic compounds, and in particular to naphthalene, benzene and benzene homologs.

Coal tar, obtained during carbonization of coal, is a highly aromatic mixture of many organic compounds. The aromaticity of coal tar depends mainly on the temperature at which coal is carbonized and on the severity (temperature, residence time) with which the hot coal gases are thermally cracked in the free space above the coal charge. Tar obtained from hightemperature coke ovens is more than 99 percent aromatic in nature, while low-temperature coal tar contains varying amounts of aliphatic compounds and higher amounts of compounds containing oxygen, nitrogen, and sulfur. Coal tar, produced either by coke ovens or coal gasifiers, may be obtained from bituminous coals as well as from subbituminous and lignitic coals. Valuable material can be obtained from the tar, although the limited amounts of these materials in the tar streams, and the availability of new synthetic methods for producing them, makes their recovery uneconomical. lt is preferable to convert the whole coal tar stream catalytically to a small number of compounds which can be obtained in large quantities and which have larger markets. Such compounds include benzene, toluene, xylenes and naphthalene.

Because of the expensive and involved procedures required in processing the tar, much of the coal tar production has been utilized as fuel.

Coal tar is generally distilled to produce two distillate fractions and a large residue. The first distillate fraction, boiling up to 200 C., is known as tar light oil," while the second fraction, boiling between 200 and 300 C., is known commonly as "chemical oil." These distillate fractions are amenable to well-known separation techniques and do not present much of a refining problem. The residue, boiling above about 300 C., and known as topped tar," contains polynuclear aromatic hydrocarbons of high molecular weight. Many of these polynuclear compounds have utility in limited quantities in the chemical and pharmaceutical industries, if they can be separated and properly refined. However, such use represents only a small fraction of the large quantity of topped tar available, topped tar in turn representing about 78 percent of the total coal tar production.

During the last decade, the marketability of topped tar steadily declined as the steel industry continued the conversion of its open-hearth furnaces to basic oxygen furnaces. While open hearths consumed a large fraction of the toppedtar production, basic oxygen furnaces do not require any hydrocarbon fuel. Consequently, it has become an extremely pressing problem to find new outlets for tar.

When polynuclear aromatic hydrocarbons are hydrogenated, the reaction proceeds in steps, resulting in a number of compounds having various degrees of saturation. For example, when anthracene is hydrogenated, the following compounds can be formed sequentially:

this series anthracene is an aromatic, tetradecahydroanthracene is a naphthene, while the three intermediate compounds have some saturated, or naphthenic, rings and some aromatic rings. 1 wish to refer here to any such partially hydrogenated polynuclear compounds as aromaticnaphthenes.

A method commonly used to reduce the molecular size of hydrocarbons is hydrocracking. ln hydrocracking, polynuclear aromatics are first partially hydrogenated to aromatic-naphthenes and the naphthenic rings of these aromaticnapthenes undergo cracking and rearrangement. The alkyl groups formed from the hydrocracking of the naphthenic rings are hydrogen deficient. Part of these groups is saturated immediately with hydrogen, while the balance is rearranged into olefins at the high hydrocracking temperature. The products of this hydrocracking are some C to C, gases and an aromatic-naphthene or a naphthene containing one less ring than the polynuclear aromatic reactant.

While prior hydrocracking processes are known which convert topped tar to salable low-boiling products in large quantities, the purpose has mostly been to produce aliphatic products such as gasoline or diesel oils. No prior process has been developed for conversion of topped tar to low molecular weight aromatics with high yields. Although such aromatics can be obtained by hydrocracking topped tar under certain conditions, increased product aromaticity and increased selectivity (high yield) are mutually exclusive in such procedure. One possible method of producing oneand tworing aromatics is to hydrocrack polynuclear aromatics under conditions maximizing the yield of oneand two-ring hydrocarbons without concern for the aromaticity of these hydrocarbons, and then to rearomatize them to the desired products. Existing art for rearomatizing oneand two-ring naphthenes and aromatic-naphthenes is quite nonselective and wasteful. The aromatization reaction would be accompanied by isomerization to five-membered rings and to other undesirable nonaromatics, and the final product would include considerable amounts of C, to C, (methane, ethane, propane and butane) gases which have only fuel value.

It is therefore an object of this invention to convert, selectively, polynuclear aromatic compounds, such as those found in topped tar, to aromatic hydrocarbons having only one or two, or if desired, three six-membered carbon rings.

Another object is to convert topped tar into low molecular weight aromatic compounds by a three-step hydroupgrading process.

A further object os to produce a maximum of naphthalene from tar with a minimum of C, to C, gases.

An additional object is to hydroupgrade, catalytically, coal tar fractions lighter than topped tar simultaneously with topped-tar to low molecular weight aromatic compounds.

it is another object to hydrorefine the polynuclear aromatic stream, and to produce oneand two-ring aromatics free of impurities of oxygen, nitrogen or sulfur compounds.

SUMMARY OF THE lNVENTlON Primarily, the invention relates to hydroupgrading topped tar, or tars of similar composition, and to this end will be described briefly immediately below.

swimme 1, 2, 3,4-Tetrahxdroanthra0e include aromatics, aromatic-naphthenes, and naphthenes containing up to three rings. The formation of one-ring naphthenes has been held to a minimum in reactor 22.

In reactor 63, the gaseous feed is treated catalytically over a chromia-alumina catalyst at a temperature of about 610 C., a pressure of 800 p.s.i.g., and at a liquid hourly space velocity of 0.5 vol./(hr.) (voL). The following simultaneous reactions occur in reactor 63:

a. Part of the twoand three-ring aromatic-naphthenes and part of the one-ring naphthenes are dehydrogenated to aromatics without ring breakage.

b. The balance of the twoand three-ring aromaticnaphthenes is hydrocracked, thereby losing the naphthenic (saturated) ring. The product of this hydrocracking reaction is an aromatic molecule containing one ring less than the aromatic-naphthenic or naphthenic reactant. The balance of the one-ring naphthenes is also hydrocracked. Hydrocracking of naphthenic rings produces C, to C hydrocarbons, which are removed as waste.

. Alkylaromatics are dealkylated stepwise to nonsubstituted aromatics.

. Compounds containing sulfur, oxygen and nitrogen,

which have not reacted in reactors 22 or 24, are

hydrogenated to hydrogen sulfide, water and ammonia respectively plus a hydrocarbon.

Residual paraffinic or olefinic hydrocarbons are hydrocracked to C to C hydrocarbon gases. Nonsubstituted aromatics pass through the reactor unreacted.

The effluent from reactor 63 flows through line 64 to cooler-condensor 65, where the aromatic products are liquefied. In order to prevent the crystallization of naphthalene, and any remaining anthracene or other crystallizable aromatics at the condensation temperature of benzene, the effluent from reactor 63 is quenched with a stream of recycled light hydrocarbons at line 64. The mixture of recycled hydrocarbons, in the case of C and C aromatic hydrocarbons, and reactor 63 effluent has a considerably lower crystallization point than the effluent alone, and can be cooled conveniently to a temperature at which most of the benzene is condensed. The cooled mixture of reactor 63 efiluent and C to C hydrocarbons is separated in high-pressure flash drum 66 to liquid aromatics and spent hydrogen. Hydrogen is exhausted from the system by way of line 67 and valve 67' and sent to the previously referred-to hydrogen purification unit. The liquid aromatics are transferred by virtue of their pressure through line 68 and valve 68' and through a stripper and a clay treater, not shown, to a fractionator 71, where they are separated into pure fractions of benzene, toluene, xylene, C to C hydrocarbons and naphthalene as shown at lines 72, 73, 74, 75 and 76 respectively. In the stripper the C to C, hydrocarbon gases, hydrogen sulfide and ammonia, which are dissolved in the aromatic product at the pressure of flash drum 66, are separated from said product. The clay treater removes color and the last traces of olefins. The bottoms contain methyl-naphthalenes and heavier aromatic hydrocarbons, which are recycled by way of line 79 to fractionator 36 where methyl-naphthalenes are distilled within the overhead and eventually dealkylated to naphthalene upon returning to reactor 63. The heavier hydrocarbons remain in the bottoms fraction of fractionator 36, and are recycled to the first step reactor 22 for further hydrocracking. An amount of C to C aromatic hydrocarbons, equal to those contained in the reactor 63 efiluent is recycled by way of lines 77, 47 and 62 to reactor 63 for further dealkylation to xylenes. The remainder of the C to C hydrocarbons from fractionator 71 is split off from line 77 to line 78 to act as quench for reactor 63 effluent at line 64.

Biphenyl can also be separated as a product in fractionator 71 instead of returning it to fractionator 35.

Fractionators 71 and 12 may each have several columns, and may be equipped with distillate cooling means as well, and the required design for obtaining the desired fractions in each case will be apparent to those skilled in the art.

In practicing the process of this invention, a batch, or once through process, would result in about 30 percent recovery of oneand two-ring aromatics from coal tar, creosote or similar tar composition. In a continuous, recycle process, as described in the example, recovery of oneand two-ring aromatics can reach as high as 70 percent to 75 percent of the total original feed, with a slightly lower yield when topped tar alone is used as feed.

In the hydrotreat reactor, catalysts other than the one named may be used. For example, sulfided nickel-molybdenum on alumina is satisfactory. Any hydrocracking catalyst may be used in hydrocracking reactor 22.

While the operating characteristics, such as temperature, pressure, hydrogen-to-feed volume ratio, LHSV, etc. given in the example, are considered optimum, considerable flexibility is permitted in these characteristics. For example, the hydrocracking step in reactor 22 can be performed at a pressure between 1,000 and 10,000 p.s.i.g., a temperature of from 400 to 550 C., an Ll-ISV of from 0.1 to 2.0 vol./(hr.)(vol.), and a hydrogen-to-feed volume ratio between 500 and 5,000. The pressure in the second, or hydrotreat reactor 74, may range from 400 to 10,000 p.s.i.g., with a permissible temperature of from 250 to 450 C. and an LHSV of from I to l0. In the third reactor step, reactor 63, the pressure may range from 400 to 1,500 p.s.i.g., with a temperature range from 560 to 660 C., an LHSV of from 0.2 to 2.0 vol./(hr.)(vol.), and a hydrogen to ring hydrocarbon mol ratio of from 4 to 12.

In a modification of the foregoing example, a BT fraction, a C to C fraction and a phenol-containing chemical oil fraction are separated by the fractionator 12 as shown in FIG. 2. The BT fraction is added to line 47, the C C fraction is added to the inlet of the hydrotreat reactor 24, and the chemical oil is hydrotreated in a separate hydrotreat reactor 54. The chemical oil from fractionator 12 is brought, via pump 52 and line 53, to hydrotreat reactor 54. Hot compressed process hydrogen is introduced into reactor 54 by mixing with the feed just prior to entry of both streams into the reactor. Reactor 54 effluent travels, via line 56, to meet the mainstream feed in line 47. ln this modification, the hydrocrackate and the C to C streams, which contain olefins but no phenols, are hydrotreated at mild conditions of about 300 C., 3,000 p.s.i.g. and an LHSV of 5 vol./(hr.)( vol.) while the chemical oil stream, which contains considerable phenols and some olefins, is hydrotreated under more severe conditions of about 400C, 800 p.s.i.g. and an LHSV of l vol./(hr.)(vol.).

Another modification of the process is the production of three-ring aromatics, along with one-ring and two-ring aromatics, if the market warrants such production. ln this case, fractionator 12 is operated in a manner to produce an overhead chemical oil distillate containing compounds boiling up to 350 C.

lclaim:

l. A process of treating a polynuclear aromatic hydrocarbon feedstock which comprises:

A. catalytically hydrocracking said feedstock in a first reaction zone in a hydrogen atmosphere at elevated temperature and thereby hydrocracking the large molecules to three-ring and two-ring aromatic-naphthenes,

B. catalytically hydrotreating the hydrocracked feedstock in a second reaction zone at a temperature, pressure and liquid hourly space velocity sufficient to saturate olefins formed in the first reaction zone, and

C. catalytically treating the thus hydrotreated feedstock in a third reaction zone at a temperature, pressure and hydrogen-to-hydrocarbon feed mol ratio sufficient to bring about simultaneously the following reactions:

a. hydrocracking three-ring and two-ring aromaticnaphthenes to two-ring and one-ring aromatics respectively,

b. dehydrogenating the three-ring and two-ring aromaticnaphthenes and naphthenes to aromatics,

c. hydrodealkylating alkyl aromatics to nonsubstituted aromatics, and

d. hydrorefining substantially all of the oxygen, nitrogen and sulfur compounds unreacted in the first and second I reaction zones.

2. A process according to claim 1 wherein the temperature, pressure and hydrogen-to-feed mol ratio in the third reaction zone are 560660 C., 400-],500 p.s.i.g. and 4-12 respectively.

3. A process according to claim 2 wherein the liquid hourly space velocity in the first reaction zone is from 0.1-2.0 vol./(hr.)(catalyst vol.).

4. A process according to claim 2 wherein the reactions in the third reaction zone are performed in the presence of a chromia-alumina catalyst.

5. a process according to claim 2 wherein the temperature and pressure in the first reaction zone are 400550 C. and LOGO-10,000 p.s.i.g. respectively.

6. A process according to claim 4 wherein the reaction in the first reaction zone is performed in the presence of a metal sulfide catalyst supported on alumina.

7. A process according to claim 1, wherein the polynuclear aromatic hydrocarbon feedstock is obtained from the carbonization or gasification of a coal of the group consisting of bituminous, subbituminous, and lignite.

8. A process according to claim 1, wherein the polynuclear aromatic hydrocarbon feedstock is a topped tar obtained from high-temperature coal tar.

9. A process according to claim 1, wherein the polynuclear aromatic hydrocarbon feedstock is a crude coal tar.

10. A process according to claim 9 wherein chemical oil and C3 to C hydrocarbons are individually separated from the feedstock, the chemical oil is first hydrotreated in an additional reaction zone under conditions sufficient to saturate the olefins and remove oxygen from phenols without appreciably saturating aromatic rings, and is then treated in the third reaction zone (C), and the C to C m hydrocarbons are treated only in the second and third reaction zones (B) and (C) respectively. 

2. A process according to claim 1 wherein the temperature, pressure and hydrogen-to-feed mol ratio in the third reaction zone are 560*-660* C., 400-1,500 p.s.i.g. and 4-12 respectively.
 3. A process according to claim 2 wherein the liquid hourly space velocity in the first reaction zone is from 0.1-2.0 vol./(hr.)(catalyst vol.).
 4. A process according to claim 2 wherein the reactions in the third reaction zone are performed in the presence of a chromia-alumina catalyst.
 5. A process according to claim 2 wherein the temperature and pressure in the first reaction zone are 400*-550* C. and 1,000-10,000 p.s.i.g. respectively.
 6. A process according to claim 4 wherein the reaction in the first reaction zone is performed in the presence of a metal sulfide catalyst supported on alumina.
 7. A process according to claim 1, wherein the polynuclear aromatic hydrocarbon feedstock is obtained from the carbonization or gasification of a coal of the group consisting of bituminous, subbituminous, and lignite.
 8. A process according to claim 1, wherein the polynuclear aromatic hydrocarbon feedstock is a topped tar obtained from high-temperature coal tar.
 9. A process according to claim 1, wherein the polynuclear aromatic hydrocarbon feedstock is a crude coal tar.
 10. A process according to claim 9 wherein chemical oil and C8 to C10 hydrocarbons are individually separated from the feedstock, the chemical oil is first hydrotreated in an additional reaction zone under conditions sufficient to saturate the olefins and remove oxygen from phenols without appreciably saturating aromatic rings, and is then treated in the third reaction zone (C), and the C8 to C10 hydrocarbons are treated only in the second and third reaction zones (B) and (C) respectively. 